1. Field of the Invention
The present invention relates to a lever transmitter for determining a filling level of liquid in a tank, with a lever arm pivotally mounted on a carrier and carrying a float and with a potentiometer for generating electric signals as a function of the pivot angle of the lever arm. The potentiometer has two sliding tracks arranged on the carrier and a contact plate arranged on the lever arm and is prestressed in the direction of the sliding tracks by means of a spring element, said contact plate having two sliding contacts connected electrically to one another and bearing on the sliding tracks.
2. Description of the Related Art
Such lever transmitters are used, in particular, for determining a filling level of fuel in a fuel tank of a motor vehicle and are known in practice. Since two sliding tracks are arranged on the carrier, there is no need for electric leads to be fastened to the lever arm. This makes the lever transmitter particularly cost-effective. In the known lever transmitter, the contact plate is of angular design, a first leg carrying the two sliding contacts and the second leg being guided in the lever arm. The spring element is designed as a leaf spring and bears on that side of the first leg of the contact plate which faces away from the sliding contacts.
A disadvantage of the known lever transmitter is that vibrations and bending forces acting on the lever arm may cause the sliding contacts to lift off briefly from the sliding tracks. Furthermore, tolerances in the height of the sliding tracks lead to the leaf spring exerting varying prestress on the sliding contacts.
The problem on which the invention is based is to design the lever transmitter of the type initially mentioned, in such a way that the sliding contacts are prestressed against the sliding tracks with as constant a force as possible.
This problem is solved, according to the invention, in that the sliding contacts are moveable relative to the contact plate in the direction of the sliding tracks.
By virtue of this design, the sliding contacts are no longer fastened rigidly to the contact plate. Tilting movements and varying prestresses of the sliding contacts are compensated by their moveability.
Consequently, tolerances in the height of the sliding tracks or bending forces acting on the lever arm do not cause the sliding contacts to lift off. Moreover, the prestressing force of the spring element is distributed largely uniformly to the two sliding contacts. In the event of vibrations, therefore, a constantly reliable force prestressing the sliding contacts against the sliding tracks is likewise ensured. The risk that the sliding contacts will lift off is markedly reduced by virtue of the invention.
According to an advantageous development of the invention, the moveability of the sliding contacts can be achieved at particularly low outlay in terms of construction if a further spring element is arranged between the contact plate and the two sliding contacts. The prestress on the sliding contacts is thereby generated by springs arranged one behind the other. Tests showed that, when fuel supply transmitters were subjected to random vibration, markedly fewer contact interruptions, particularly in the range of between 0.1 and 0.5 ms, occurred. Longer contact interruptions of more than 1 ms due to liftoffs of the sliding contacts no longer occurred at all. The lever transmitter according to the invention is therefore reliably protected against liftoffs, particularly in the case of vibrations often occurring in motor vehicles.
According to another advantageous development of the invention, the contact plate and the sliding contacts are designed particularly simply in terms of construction if the sliding contacts are arranged in each case on spring tongues.
According to another advantageous development of the invention, in the case of bending forces acting on the lever wire, it is possible in a simple way to ensure that the two sliding contacts are pressed against the sliding tracks, if the sliding contacts are arranged on ends of a crossbar and if the crossbar is connected centrally, via a bending spring, to a guide region of the contact plate. In this case, the prestress on the sliding contacts is more uniform, the lower the spring constant of the bending spring is.
Plastic deformation of the resilient arms or of the bending springs due to high bending forces can be avoided in a simple way if the contact plate has stops for limiting the range of movement of the sliding contacts.